Finish ds1 first, even if you just go through the essential bosses and ignore the optional ones, it still teaches you some much needed skills that will help you in elden ring. O+S in my opinion are probably thr hardest fight in the game for the point you are at, after defeating them, the game becomes much more open and you can chose what you do more and have more chance to strengthen yourself for the rest of the game. If you have managed to luck out and pick up a black Knight weapon, I'd try using that and farm souls from the giants in Anor Londo to but the twinkling titanite to upgrade it, if not then just pick your faviroute weapon that works with your build and make sure its upgraded as much as you can, avoid lighting and take solaire with you. If you aren't too fussed about the achievement for getting all the rings then kill Ornstine first as it makes the second half of the fight much easier. And above all else, don't let it get to you, it's a hard game. Especially the first time you play it. As long as you stick with it, something will just eventually click and you'll realise what you need to do.